Here's some early recording by rock stars, some deservedly underappreciated and some not so.
Neil Young,, "It's My Time." It was 1966 and Young had just joined the Toronto band Mynah Birds. Also members of the group were Rick James, Goldie McJohn (Steppenwolf), and Bruce Palmer (Buffalo Springfield). This tune was recorded by VIP Records (a Motown subsidiary) but was not released until 40 years later.
Sly Stone, "'You've Forgotten Me." Sly was still a teenager when he fronted the Bay area band The Viscaynes as "Danny [Sly] Stewart and recorded this bit of doo-wop, circa 1961.
Stephen Sills, "High Flying Bird/" The Au Go Go Singers were a nine-member folk ensemble in 1964. Also in the group was Richie furay (Buffalo Springfield).
Simon and Garfunkel, "Hey, School Girl." In 1957, the duo were known as Tom & Jerry when they recorded this. Back in those days, they also did a lot of demos and issued singles under a number of aliases.
Art Garfunkel, "Beat Love." This one is from 1959 under the name "Artie Garr,"
Paul Simon, The Lone Teen Ranger." In 1962, Paul Simon used the name Jerry Landis on this song with Tico and the Triumphs.
Lou Reed, "Merry-Go-Round." He was still Lewis, rather than Lou, in 1962 when he recorded this unreleased song in 1962.
Procol Harum, "Poison Ivy." The Paramounts released this cover of The Coasters' hit in 1963. Members of the Paramounts included future Procol Harum members Gary Booker, Robin Trower, Chris Coppin, and B. J. Wilson.
Roger McGuinn, "Beach Ball." McGuinn co-wrote and played guitar on this single from the City Surfers in 1963.
Cass Elliot & Denny Doherty, John Sebastian & Zal Yanovsky, "I Don't Wanna Know." The future members of The Mamas and The Papas and The Lovin' Spoonful, along with Jim Hendricks (who later formed the Lamp of Childhood and wrote "Summer Rain" for Johnny Rivers) were The Mugwumps. After their later groups made it big, Warner Brothers released their only album, sans John Sebastian in 1967. Sebastian did sing in this 1964 single, though.
Jimi Hendrix, "Testify." This 1964 outing with The Isley Brothers may have been Hendrix's first recording. (the other contender for Hendrix's first recording is "My Dairy," a song recorded with Rosa Lee Brooks.")